The present invention relates to a rubber composition for tire treads and a pneumatic tire having a tread made of the rubber composition, and particularly relates to a rubber composition for tire treads significantly improving tires in grip performance on wet road with remaining low fuel consumption of automobiles and a pneumatic tire having a tread made of the rubber composition.
In recent years, tires for automobiles have required performances, such as the controllability in driving, abrasion resistance, riding comfort as well as the low fuel consumption. Steps taken to achieve such performances include improving braking and driving on wet roads at high driving speed, improving controllability in driving by increasing the grip force on road surfaces, increasing cornering performance by increasing the block stiffness of the tire tread pattern to inhibit the tire from block deformation at cornering, and inhibiting hydroplaining from occurring by inhibiting groove parts on tire tread from deforming to achieve smooth draining. Recently, to satisfy these requirements, tires having an increased grip performance on wet road surface are provided by using rubber compositions obtained by mixing silica with SBR of a high styrene unit content for tire treads.
However, although the rubber compositions for tire treads mentioned above provide an increased grip force at a low temperature range of at most 15° C. of road surface, they do not provide a sufficient grip force on wet or semi-wet road surface. Rubber compositions containing silica decrease in stiffness and decrease significantly in grip force with the repetition of driving. Rubber compositions containing silica have problems such as an increase in Mooney viscosity and a decrease in processability. For example, when extruding, the silica disperse insufficiently in the rubber compositions.
Various proposals have been traditionally made to solve these problems mentioned above. Rubber compositions which are effective in improving the grip performance of tires improve, for example, Japanese unexamined patent publication No. 133375/1995, and No. 311245/1996 which disclose rubber compositions obtained by mixing calcined clay with diene rubbers Japanese unexamined patent publication No. 3373/1996 discloses rubber compositions obtained by mixing vulcanized rubber powders comprising diene rubbers and kaolinite with specific kinds of diene rubbers. As rubber compositions showing the same effects, Japanese unexamined patent publication No. 59893/1996 discloses rubber compositions obtained by mixing specific kinds of inorganic powders and carbon black with SBR comprising specific amount of styrene units, and Japanese unexamined patent publication No. 149954/1995 and No. 31250/1997 disclose rubber compositions obtained by mixing kaolinite base clay with diene rubbers having a specific ratio of 1,2-bond in the butadiene units part.
However, no rubber composition provided shows an excellent wet grip performance with a low heat build up characteristic and without a decrease in processability and a decrease in abrasion resistance.